Impulse duration responsive or indicating device



May 7, 1946. 0, T. FRANCIS I 2,399,663

IMPULSE DURATION RESPONSIVE OR INDICATING DEVICE Filed Aug. 24, 1943 I 527m '& Qum

i l l l f l l l l l 10 v Patented May 7, 1946 UNITED sTArEs PATENT OFFICE IMPULSE DURATION RESPONSIVE on v I mmca'rmo nnvrcn Oliver T. Francis, Renville, Minn.

Application August 24, 1943, Serial No. 499,757

14 Claims. (01. 177-353) greater exactitude in measuring said time length This invention relates to impulse duration responsive or indicating'devices, and more particularly to means selective to signals of predetermined time durations.

Various circuits have been proposed which are selective to signals of predetermined time durable of differentiating between signals of widely diflerent time lengths such as a dot" or a dash, a dash" being three times the length of a dot. It is an object ofthis invention to provide a vacuum tube circuit which shall be highly selective in its response to signals of predetermined duration only; Another object is to provide a circuit in which any one oi a large number of electro-responsive devices may be operated by a may be made possible.

- Another object is to provide a novel means for initiating a pulse when the direction or charge of a condenser changes to initiate a voltage indicating means for indicating the value of the charge on said condenser when said direction changes. 1

Another object is to provide a vacuum tube having an input and an output circuit, a resistance shunted by a condenser in said input and output circuit, the primary of a transformer connected in said output circuit, means for impressing a negative potential on said input circuit for a period of time to be determined, means for indicating the voltageacross said condenser, and means for applying a pulse from the secondary of said transformer to said indicating means to corresponding one of a large number of time length signals.

Another object is to provide a circuit responsive to electrical currents of predetermined time length regardless oi the magnitude of said currents thereby preventing the effects of shock excitation from high voltages such as static in conventional amplifier circuits.

Another object is to provide a duration indicating means wherein the signal voltage initiates a predetermined rate of change ,in voltage be; tween the plates of a condenser for the duration of said signal and means for indicating the voltage or the remaining charge on said condenser at the end of said signal for indicating the duration of said signal.

Another object is to provide a time measuring device wherein the charge on a condenser is changed at a predetermined rate for the duration of said time and means of indicating the voltage of the charge remaining on said conrender it operative at th instant said condenser changes from a discharging to a charging condition.

In Fig. 2 of U. S. Patent No. 2,300,664, I illustrated a receiver which would respond to two input pulses separated by a predetermined duration of time, and a photoelectric means for varying said duration in accordance with a secret record. It is an object of my present invention to illustrate a similar receiver which will respond to an input signal of predetermined duration and r a photoelectric record for varying said duration.

Another object is to provide means for changing the signal time length of which an electroresponsive device is responsive in accordance with a record. Another object is to provide photoelectric means for varying the signal duration to which. an electro-responsive device is responsive.

These and other objects of the invention will become obvious from the following description, claims, and figures, wherein:

Fig. 1 illustrates a circuit diagram for an impulse duration responsive or indicating device comprising my invention.

Fig.2 shows voltages of several time lengths applied to the device.

to be determined, it has in the pest'been dim-.

cult to indicate the exact value of said charge without a plurality of stages of vacuum tube a-mpliilcation. provide a single circuit wherein aplurality of condensers may be successively discharged whereby the "scanner voltage" may be eflectively multiplied by the number or said condensers, and

It is an object of this invention to.

' Referring to Fig. 1, the apparatus to the left or dotted line I comprises means for producing pulses of any desired time length as shown in Fig.2(B.C,orD.)

The apparatus to the right or dotted line I illustrates means responsive to pulses of duration shown in' Fig. 2 (B, C, or D) and rendering corresponding electro-responsive devices 2, 3, or 4 responsive. I r

The means for producing pulses of varying time length such as shown in Fig. 2 (B, C, or D) combattery 58.

prises an alternating current generator 8 in the primary of transformer 8. One terminal of the secondaryof this transformer is connected to a The signals produced by vacuum tube 8 are of varying time length as shown in Fig. 2 (B, C, or D) and are impressed on; the receiving apparatus to the right of dotted line I, the grid of vacuum tube I2 being connected to the anode of tube 8.

The anode of vacuum tube I2 is connected through primary of transformer to the positive terminal of battery I8. The cathode of vacuum tube I2 is connected through resistance I4 in parallel with condenser II to the negative terminal of battery II. In parallel with resistance I4 is a gas discharge device I8 and resistance II.

The output circuit of vacuum tube I2 may be traced from ground 24, to positive terminal of battery I8, primary of transformer I8, anode to cathode of tube I2, resistance I4, negative terminal of battery I8 to ground 24. Condenser I8 connects the cathode of tube I2 to ground. The input circuit of tube I2 may be traced from grid of tube I2, to resistance II, positive terminal of battery 88, ground 24, negative terminal of battery I8, resistance I4, to cathode of.tube- I2. It

is therefore apparent that resistance I4 together with a portion of battery I8 in parallel with condenser I8 are connected in the common input and output circuit of tube I2. Due to the well known fact. that the anode to cathode voltage drop of a gas discharge device such as I8, is substantially independent of current flow, variations in voltage across resistance I4 produced by varying the potential of the grid of tube I2, appear across resistance Il, while the voltage drop across gas discharge device I8 remains substantially constant.

Vacuum tube 28 may be of the pentlgrid converter type. Its first grid is connected to potentiometer I8 near the negative terminal of that potentiometer through high resistance 48. Its anode grid is connected through resistance 88 to the positive terminal of battery I8, and to the negative terminal of battery I8 through potentiometer 88. The third and fifth grids are connected together internally and to a positive terminal of battery I8. The fourth grid is connected to a midpoint of potentiometer 88. Its cathode is connected to ground 24. Its anode is connected through an electro-responsive device in the form of gas discharge device 2 to the cathode of vacuum tube I8. The ,anode of tube I8 is connected to the positive terminal of battery I8. One terminal of secondaryof transformer I8 is connected to grid of tube I-8 through resistance 81. The other terminal of said secondary is connected to ground 24.

Vacuum tube 22 may .alsobe of the pentigrid converter type. Its first grid is connected to potentiometer 2| through resistance 44, at a point more positive than the point where the first grid 2,soe,cca

of tube 28 is connected to potentiometer I8. Its anode grid is connected through resistance 28 to the positive terminal of battery I8 and through potentiometer 28 to negative terminal of battery I8. 1 Its third and fifth grids are connected together internally and to a positive terminal of battery I8. Its fourth grid is connected to a midpoint on potentiometer 28 through resistance 48.

Its cathode is connected to ground 24. Its anode is connected through electro-responsive device 8, in the form of a gas discharge device, to the cathode of vacuum tube I8.

Vacuum tube 21 may be a high mu triode. Its grid is connected through resistance 28 to midpoint of potentiometer 28. ,Its cathode is connected to ground. Its anode is connected through resistance 28 to the positive terminal of battery I8.

Vacuum tube 4I may be a triode. Its anode is connected to the positive terminal of battery I8. Its grid is connected to the anode oftube 21.

Its cathode is connected through resistance 88 r in parallel with condenser 28 to negative terminal of battery I8; In parallel with resistance 88 is .Voltage dividers 2| and I8 are in parallel with device I8.

gas discharge device 88 in series with resistance 8|. Potentiometer 82 is in parallel with gas discharge device 88.

Vacuum tube 84 may also be of the pentlgrid converter type. Its cathode is connected to ground 24. Its first grid is connected through resistance 88 to a point on potentiometer 82. Its anode grid is connected to. positive terminal of battery I8, through resistanik 42, and to negative terminal of battery I8 through potentiometer 85. Its third and fifth grids are connected together and to a positive terminal of battery I8. Its fourth grid is connected to a point on potentiometer 85. Its anode is connected through electro-responsiv device 4 in the form of a gas discharge device, to the cathode of vacuum tube I8.

The output circuit of vacuum tube I8 may be traced from the positive terminal of battery I8, to the anode and cathode of tube I8 and through three parallel paths to ground 24. i} first of these paths includes gas discharge device 2, the anode and cathode of tube 28 and ground 24. A second path includes gas discharge device 8. the anode and cathode of tube 22, and ground 24. A third of these paths includes gas discharge device 4, the anode and cathode of tube and ground 24. The input circuit of vacuum tube I8 can be traced from its grid through resistance 31,

secondary of transformer I8, to ground 24, and to in parallel with resistance 88. The input circuit of tube 4I can be traced from the grid oftube.

the anode to cathode of vacuum tube- 4|, 2'I, ground 24. negative terminal of battery "L resistance 38 to cathode of tube 4|. noted that resistance 88 in parallel with condenser 28 is in the common input output circuit of tube 4|. 1

The output circuit of tube 21 can be traced from the positive termina1 ofbattery I8, to resistance 28, anode to cathode of tube 21, ground 24, to positive terminal of battery I8. The input circuit of tube 21 can be traced from the grid Itwillbeof tube 21, to resistance 26, midpoint on potentiometer 23, negative terminal of battery l0, to ground and cathode of tube 21.

The first grid circuit of vacuum tube 20 can be traced from said first grid, through resistance 43, midpoint on voltage divider l8, resistance l1, negative terminal of battery It, to ground 24, and cathode of tube 20. The anode grid circuit of tube 20 can be traced from the positive terminal of battery ll resistance 38, anode grid of tube 20, cathode of tube 20, ground 24, to positive terminal of battery Ill. Potentiometer 39 is connected from said anode grid to the negative terminal of battery II. The fourth grid circuit of tube 20 can be traced from the midpoint of potentiometer 39, negative terminal of battery l0, ground 24, to cathode. of tube 20. Anode. space current can pass through tube 20 onlywhen none of its grids are sufficiently negative to block said anode current. As thefirst grid of tube 20 tends to assume a less negative potential this anode space current increases, but the anode grid space current through resistance 33 also increases with the resulting lowering of potential of said" anode grid. A portion of this lowered potential is applied to the fourth grid of tube 20 by potentiometer 39, and when said fourth grid is suiiiciently negative anode space current through tube 20 is blocked by the neg-. ative potential on its fourth grid. It follows that tube 20 is conductive to anode current for one value of input voltage applied to its first grid. The value of this input voltage is slightly more negative than ground, that is the voltage that the cathode of tube I2 is above ground 24, minus the D. C. voltage drop across the portion of potentiometer i8 between the cathode of tube I 2 and the point where resistance 43 is. connected to potentiometer l8.

Similarly vacuum tube 22 is conductive to only one value of voltage applied to its first grid through resistance H, by potentiometer 2|, which is connected between the cathode oftube l2 and the negative terminal of gas discharge device it. By varying the point on potentiometer 2| where resistance 4% connects tube 22 is conductive to anode space current for a difierent value of potential of cathode of tube l2 than tube 20 is conductive. The'function of potentiometers l8 and 2!! is therefore to render tubes 20 and 22 conductive to anode space current for diiferent potentials of the cathode of tube l2. Similarly the function of potentiometer 32 is to render tube 34 conductive to anode space current when the cathode of tube 4! assumes desired potential.

A photo-electric cell 45, controlled by light 41 a of tube l2, and means for varying said duration 7 grid of tube .8 sufliciently positive for tube 8 to through screen 46 is connected between the variable contact of {potentiometer l8 and negative terminal of gas discharge device IS. The potential of cathode of tube 12 for which anode spacecurrent wil1 flow-in tube 20 can also be controlled by variably illuminating photoelectric cell d5. Since the anode of cell 45 is connected to the variable contact of potentiometer l8 and the cathode of cell 45 is connected to the neg-' ative terminal of potentiometer I8, when cell 45 is strongly illuminated increased current flows through the upper portion of this potentiometer, the-anode to cathode of cell 4 5 to the negative terminal of potentiometer l8. This results in an increased voltage drop between the cathode of tube l2 and the grid of tube' 20,-with the result that anode space current will flow through tube 20 when the cathode of tube I2 is at a higher potential than 11'. cell 45 were not illuminated.

Since the amount of light from light 41 incident on photoelectric cell "is dependent on flow through tube 20 is controlled'by the 'transy parency of record 46. Since this record, can be kept secret communication with my device can be kept secret. The general method of intro ducing secrecy into communication by photoelectric records is described in detail in my U. S.

Patent No.'2,300,664 to which reference is made.

In Fig. 2 of that patent I illustrated a receiver responsive to pulses separated by a predetermined duration and varied said duration with a secret photoelectric record; In my present invention I have illustrated a load in the form of a gas discharge device 2, illuminated by pulses of one duration of negative voltage applied to the grid by a secret record 46.

In operation, alternating current generator 5 may produce a sine wave current of any convenient frequencysuch as 60 cycles per second. The voltage of this current is changed to any desired voltage by secondary of transformer 13. This desired voltage may be diagrammatically shown in Fig. 2 (A). Since vacuum tube 8 is a high mu tube current flows through resistance ll only when the grid of tube 8 is positive (or slightly negative). Resistance 1 being several megohms the grid of tube 8 never assumes a high positive potential. Potentiometer 9 biases grid of tube 8 negative past its cutofl except for the brief period of time in the cycle that transformer 6 throws conduct current. This time can be varied by varying the point where secondary or transformer 8 connects to potentiometer 9. In Fig. 2 (B) the connection'to potentiometer 9 is so near the negative of battery 50 as to permitsecondary of transformer 8 to impress a positive potential on grid of tube 8 for only a small part of the cycle and the voltage of anode 8 is reduced for only this small part of the cycle.

In Fig. 2 (C) potentiometer 9 impresses less negative voltage on grid of tube 8. Consequently the voltage of its anode is reduced for a period or time in the cycle.

In Fig. 2 (D) potentiometer 9 impresses much less negative voltage on grid of tube liwith result that its anode voltage is reduced for a period of nearly one-half the cycle of the voltage shown in Fig. 2 (A). When negative pulses of time length shown in Fig. 2 (B) are impressed on the grid of vacuum tube l2, condenser I5 discharges only slightly through resistance l4. This discharge is sufllcient to permit the first grid of vacuum tube 20 to assume a slightly negative potential (light longer 41 being extinguished and photoelectric cell 45 I having a substantially, infinite resistance). As

the grid of tube I2 tends again to go positive secondary of transformer l3 impresses a positive pulse onthe grid of tube H with the result that current -flows from positive terminal of battery l0, anode to cathode of tube l9 elec-- by its fourth grid by potentiometer 23. The

- extent as to permit the first'grid of tube 22 to assume a slightly negative potential and when grid of tube i2 again tends to assume a positive potential and secondary of transformer 13 impresses a positive pulse on grid of tube l9, current flows from positive terminal of battery l0, anode to cathode of tube l9, electro-responsive device 3, anode to cathode of tube 22 to ground 24. Current cannot fiow' through electro-re- "sponsive device 4, since while the grid of tube 21 is thrown positive and the grid of tube 4| negative the RC time element of condenser 29 resistance 30, is suiiiciently large that sumcient of the charge on condenser 29 does not leak of! to permit first grid of tube 34 to assume a slightly negative potential. Electro-responsive device 2 is also inoperable at this instant since the first" grid of tube 20 is sufliciently negative to block current through device 2.

If pulses of longer negative duration such as shown in Fig. 2 (D) are applied to grid of tube. l2, sufilcient of the charge on condenser l5 leaks oif to permit the first grids of tubes 20, 22 to block current through devices 2, 3, but grid of tube 21 is thrown positive and grid of tube 4| negative for a period of time sumcient for charge on condenser 29 to leak off sumciently to permit first grid of tube 34 to assume a slightly negative potential. 'When the secondary of transformer l3 impresses a positive pulse on grid of tube |9 at completion of signal shown in Fig. 2 (D) current fiows from positive terminal of responsive device 4, anode to cathode of tube 34 to ground 24.;

To further illustrate the functioning of my circuit the constants of a circuit used in tests are given. It is to be understood that my invenstants or even to the particular circuit arrangement of parts illustrated, the invention being defined in the appended claims. 2

Generator 5 was the 110 A. C. sixty cycle power lines. The voltage across the secondary of transformer 8 was 35 volts. Potentiometer 9 was of one megohm value, and was connected between the negative terminal of battery 50 and a point thereon 45 volts more positive. Positives of batteries III and 50 were plus 135 volts. Their negative terminals were minus 90 volts each. Vacuum tubes 8 and 21 were type 2A6. Tube I! was type 2A5, connected as a triode. Tubes 20, 22, 34 were type 2A7. Tubes 4|, l9 were type 2'7 Resistances l'l were a half megohm each. Condenser It was a tenth microfarad. Potentiometers fl, 35, 89 were 2.0 meghoms each and a connection was made to the center tap of each, Potentiometers l8, 2|, and 32 were megohms eachQ Resistances l, 28, 40 were megohms each. Resistances 38, 44, 42 were 10 megohms each. Gas discharge devices I3, 33 maintained a constant voltage drop of 135 volts across each. Electro-responsive devices in the form of gas discharge devices 2, 3, 4 became conductive at 70 volts each. Transformer it was a 5 to 1 step-up audio transformer. i

In this experimental circuit when the len th of signal was lengthened by varyin potentiometer 9 it was possible to selectively "tune in eight diflerent signal pulse lengths with a tube such as 20 whose grid was connected to potentiometer I! (light 41 being extinguished). Then as the signal was further lengthened it was posr si-ble to "tune in" five additional signals by a tube 34 whose grid was connected to potentiometer 22. Since the maximum negative signal pulse as shown in Fig. 2 D) was less than .01 second it is obvious that I was able to discriminate between signal pulse lengths diil'ering in length by only a few ten thousandths of a second. 4

when the movable contact of potentiometer l8 was set near its middle point, and Mt 41 lighted the length of signal -to-which tube 24 responded was dependent on the transparency of screen 46. I

It is further to be noted that the sine wave Fi 2(A) was not saw toothed" and that more linear response is possible with "saw toothed" waves.

It is obvious to those skilled in the art that the invention may take widely different forms from those illustrated without departing from its spirit and it is to be limited in scppe only as defined in the following claims,

whatlclaimis:

1. an impulse duration indicating device responsive to negative signal. voltages of one predetermined time length only, comprising: a vacuum tube having an input and an output circuit,. a resistance shunted by a condenser connected in said input and said output circuit, the primary of a transformer and a source .of current connected in said output circuit. a Positive battery l0, anode to cathode of tube l9, electrotion is not limited to the values of these conbiasing source conne ted in said input circuit to permit said source;of current to charge said condenser, means for applying said signal voltages to said input circuit, to blockcurrent flow into said condenser for said time length, indicating means responsive only to voltage of the magnitude of said voltage remaining on said condenser at the end of said time length, and means for applying a pulse from the secondary of said transformer to said indicating means to render it operable only at the end of said time length.

' output circuit, a resistance shunted by a condenser connected in said input and said output circuit, the primary of a transformer and a source of current connected in said output circuit, means for applying said signal voltages to said input circuit to block current flow into said condenser for said time length, means for indicating the voltage across said condenser and 'means for applying a pulse from the secondarv of said transformer to said indicating means to render itoperative only at the end of said time length.

3. A vacuum tube circuit responsive to negative signal voltages of predetermined time length only, comprising: a vacuum tube having an in 2. A circuit responsive to negative signal voltages of predetermined time length only, comprising: a vacuum tube having an input and an grid, a second'grid for controlling current flowto said cathode from said anode, means for applying voltage variations across said condenser to said first grid, means for applying voltage variations across said anode grid loadto said secasaaees for a period of time, means for producing, only ond grid to permit anode current to flow for voltages of said magnitude, a third vacuum tube having a plate, filament, and controlelectrode, said plate, and filament being connected in series with said load in the circuit of said anode, means for applying a pulse from. the secondary of said transformer to said control electrode at the end of said time length to permit current to now through said anode load, and means for indicating voltage variations across said anode load.

4. A circuit for operating any one of a plurality of electro-responsive devices by input negative signal voltages of different time lengths, comprising: a vacuum tube having an input and an output circuit, a resistance in parallel with a condenser connected in said input and saidoutput circuit, the primary of a transformer and a uum tube having an anode, a cathode and a control electrode, a plurality of' voltage magnitude operated vacuum tubes, each having a plate, filament, an anode grid, and a plurality of grids, means for connecting each of said electro-respon'sive devices and the anode and cathode of a separate one of said voltage magnitude operated tubes in the common anode, control electrode circuit of said second vacuum tube, means for applying voltage variations across said condenser to afirst of said grids in each of said voltage magnitude operated tubes, a separate anode grid load in the circuit of each of said anode grids, means for applying voltage variations of said anode grid of each of said last tubes to a second of said grids of said last tubes to permit plate current to new in each of said last tubes for one value of input voltage, means for biasing said first grid ofeach of said tubes in difierent magnitude, and means for applying a positive voltage pulse from the secondary of said transformer to said control electrode at the end of said time lengths.

5. A circuit for operating any one of a pluity of electro-responsive devices by a negative input voltage of one of a plurality'of different time lengths, comprising: ,a vacuum tube having an input and an output circuit, a resistanceand a condenser connected in parallel insaid input and said output circuit, the primary of a transformer and a source of current connected in said output circuit, means for applying said input voltage to said input circuit, a voltage magnitude discriminating device comprising units each remon circuit of said anode, and said control electrode, and meansfor applying a positive pulse from the secondary of said transformer to said control electrode: to permit current to flow through one of units depending on thevoltage remaining across said condenser at the end of said input voltage.

6. In a circuit, having indicating devices operable at different time lengths subsequent to a time reference point, a plurality of condensers, means for producing a change in voltage across a first of said condensers at a predetermined rate from the expiration of said period, a change in voltage across a second of said condensers'at'a second predetermined rate, a first of said indi eating devices operable at a predetermined voltage across said first condenser, a secondof said indicating-devices operable at a predetermined voltage across said second condenser, means whereby said first of said indicating devices operates, means whereby said second of said devices operates, connections from said first indicating device to the first of said condensers, and connections from said second of said indicating devices to a second or said condensers.

7. In a circuit having indicating devices operable at diiferent time lengths subsequent to a time reference point, a first vacuum tube having an input and an output circuit, a resistance in parallel with a condenser connected in said input and said output circuit, a second vacuum tube having an input and an output circuit, a second resistance in parallel with a second con denser. connected in said input and said output.

negative voltage to the input circuit of said sec-" ond of said tubes for a period subsequent to the beginning of said time length to the end of said time length, and means for applying a pulse from the secondary of said transformer to said indicating devices to render said devices operable only at the expirationof said time length.

8. A circuit for receiving negative signal voltages of one predetermined time length only comprising; a vacuum tube having an input and an output circuit, a resistance in parallel with a con-- denser connected in said input and said output circuit, the primary of a transformer connected the magnitude of the voltage remaining on said condenser at the end of said time length, means for applying a pulse from the secondary of said transformer to said indicating means to render H it operative only at the end of said signal voltages, a photoelectric record, and means for changing the magnitude of voltages to which saidindicating means is responsive with said rec 0rd, to vary the time length of signal voltages to which said circuit will respond,

9. In a circuit for receiving a signal, a source of.voltage variable with a duration of time, a vacuum tube having an input and an output circuit, said input and said output circuit including a plurality of parallel paths, each of said paths comprising an electro-responsive device, and a unit, each of said units being operable at a difierent magnitude of said variable voltage, and

6 I b means controlled by said signal for applying a positive pulse to saidinput circuit to permit useiul current to pass through one of said devices, depending on the length of said duration.

10. A signal duration indicating device comprising: a vacuum tube having an input and an output circuit, a source of current connected in said output circuit, a resistance in parallel with a condenser connected in said input and said output circuit, a positive biasing source connected in said' input circuit to permit charging of said condenser by space current flowing in said output circuit, means for applying a negative voltage to said input circuit to block said space w denser connected in said input and said output circuit, a source of current and the primary of a transformer connected in said output circuit, a"

positive biasing source connected in said input circuit to permit chargeot said condenser by space current flowing in said output circuit, means for applying a negative voltage to said input circuit to block said space current for said duration, means for indicating the charge remaining on said condenser at the end of said duration, said last means comprising a vacuum tube voltmeter having a grid circuit and a plate circuit, means for applying the voltage across said condenser to said grid circuit, a third vacuum tube having an anode, a cathode and a, control electrode, means for connecting said anode, said cathode, and said source 01..current in said plate circuit, means for applying a negative potential to said control electrode to block current flow in said plate circuit, means for applying a positive pulse from the secondary of said transformer to said control electrode to permit current to flow having a separate resistance and condenser connected in parallel in its common input and output circuits, a source of current connected in the output circuit of said tubes, a podtive biasing source connected in the input circuit of each of said tubes to permit charge 01' said condensers by current flowing in the output circuits of said tubes, means for applying a negative voltage to the input circuit of a first of said tubes to block current flow into a first ot said condensers for I said duration, means for applying a negative voltage to the input circuit of a second of said tubes to block current flow into a second of said condensers when said first condenser has discharged below a predetermined magnitude, and

of time, means for changing the charge of a sec; 1 0nd of said condensers at a second predetermined rate, only from the end of said period to the end of said duration, means for applying the voltage across said second condenser to said indicatin device, and means for rendering said device operative only at the expiration of said duration.

14. A signal duration indicating device comprising: a vacuum tube having an input and an output circuit, a source of current connected in said output circuit, a resistance in parallel with a condenser connected in said input and said output circuit, a positive biasing source connected in said input circuit to permit charging of in said plate circuit at'the end of said duration,

and means for indicating current flow in said plate circuit. a

12. A signal duration indicating device comprising: a plurality of vacuum'tubes having input g and output circuits, each 01' said vacuum tubes said condenser by space current flowing in said output circuit, means for applying a negative voltage to said input circuit to block said space current for said duration, means for indicating the charge remaining on said condenser at the end of said duration, a, record, and means for changing the voltage to which said means for indicating said charge will respond in accordance with said record. a

- OLIVER T. FRANCIS. 

